A liquid-jet image forming apparatus such as an inkjet recording apparatus uses one or more recording heads for jetting ink droplets to form an image. A liquid-jet image forming apparatus is used, for example, for a printer, a facsimile machine, a copier, a plotter, or a multifunction copier having functions of them. Such a liquid-jet image forming apparatus jets ink droplets from its recording heads onto paper and thereby forms (records or prints) an image on the paper. There are roughly two types of liquid-jet image forming apparatuses: a serial-type image forming apparatus including a recording head that jets ink droplets while moving in the main-scanning direction to form an image; and a line-type image forming apparatus including a line-type recording head that does not move when jetting ink droplets to form an image.
In the present application, a liquid-jet image forming apparatus refers to an apparatus that forms an image by jetting ink droplets onto a recording medium made of paper, thread, fabric, textile, leather, metal, plastic, glass, wood, ceramic, etc. Also, “image forming” indicates not only a process of forming a meaningful image such as a character or a drawing on a recording medium, but also a process of forming a meaningless image such as a pattern on a recording medium (or a process of just jetting ink droplets onto a recording medium). “Ink” refers not only to an ink (colored liquid) in a general sense, but also to any liquid usable for image forming such as a recording liquid or a fixing liquid. Further, “paper” refers not only to a recording medium (recording paper) made of paper, but also to any recording medium such as an OHP sheet or a fabric to which ink droplets can adhere.
Such liquid-jet image forming apparatuses (hereafter, simply called “inkjet recording apparatuses”) are being continuously upgraded to improve the quality of color images formed with multiple color inks, to increase the drive frequency of a liquid-jet head, and to increase the number of nozzles on each recording head to improve the recording speed.
Meanwhile, an inkjet head used as a liquid-jet head sometimes develops a problem called “ink-jetting failure” where a nozzle of the inkjet head becomes unable to jet ink droplets (such a nozzle is called a “failed nozzle”). The ink-jetting failure occurs, for example, because of dust entered into a nozzle during manufacturing, degradation of a nozzle due to long-term use, or degradation of a device for causing ink ejection. Particularly, ink-jetting failure due to degradation of a device for causing ink ejection may randomly occur during the service life of an inkjet recording apparatus.
Also, there is a case where a nozzle is not completely dysfunctional, but the direction of an ink droplet jetted from the nozzle deviates greatly from a desired direction (hereafter called an “inkjet skew”) or the size (amount of ink) of an ink droplet jetted from the nozzle greatly differs from a desired size (hereafter called “variation in droplet size”). A nozzle degraded to such an extent as to greatly reduce the quality of an image cannot be used for image recording and is therefore substantially identical with a “failed nozzle”.
FIG. 31(a) shows a pattern recorded correctly, FIG. 31(b) shows an example of complete ink-jetting failure, FIG. 31(c) shows an example of an inkjet skew, and FIG. 31(d) shows an example of variation in droplet size. In the present application, nozzles unable to correctly record images due to various causes are collectively called “abnormal nozzles”.
Abnormal nozzles have been considered as a minor problem because instances of abnormal nozzles can be reduced by improving the manufacturing environment. However, in current inkjet recording apparatuses where the number of nozzles on each recording head has been increased as described above to improve the recording speed, the problem of abnormal nozzles are not negligible. For example, manufacturing high-quality recording heads including no abnormal nozzle or designed to prevent occurrence of abnormal nozzles require higher production costs which lead to higher prices of the recording heads.
Abnormal nozzles may cause defects such as a white stripe in an image.
Patent documents 1 and 2 disclose a correction method employing multiscan recording to fill a white stripe caused by an abnormal nozzle. In multiscan recording, an image is recorded by scanning a corresponding area of a recording medium multiple times with a recording head. Therefore, the white stripe can be filled by using a normal nozzle during multiple scanning passes.    [Patent document 1] Japanese Patent Application Publication No. 5-309874    [Patent document 2] Japanese Patent Application Publication No. 2001-63008
However, to improve the recording speed of an inkjet recording apparatus, it is preferable to employ “one-pass recording” where an image is formed by scanning a recording medium only once. Accordingly, with one-pass recording, it is not possible to fill or cover (make inconspicuous, obscure, or conceal) a blank in an image caused by ink-jetting failure using a normal nozzle as in multiscan recording.
Also, even with multiscan recording, there are cases where it is difficult to fill a blank caused by abnormal nozzles due to the positions and/or number of the abnormal nozzles. For example, in a multiscan recording mode where the number of passes is comparatively small, nozzles usable as substitutes for abnormal nozzles are limited and the workload of the substitute nozzles increases. Also, in some cases, substitute nozzles may not be available due to characteristics of the recording head or drive waveform design.
Patent documents 3 through 7 propose a different correction method where a blank caused by an abnormal nozzle in highlights (low density area) of an image data is covered by increasing the density of pixels near the blank, and a blank in a dark area of an image data with saturation density is covered using dots having a different color but a similar brightness.    [Patent document 3] Japanese Patent Application Publication No. 2002-19101    [Patent document 4] Japanese Patent Application Publication No. 2003-136702    [Patent document 5] Japanese Patent Application Publication No. 2003-136763    [Patent document 6] Japanese Patent Application Publication No. 2003-136764    [Patent document 7] Japanese Patent Application Publication No. 2003-205604
However, even the correction method disclosed by patent documents 3 through 7 is not applicable to all situations. For example, image data that can be increased in density are not always available near unfilled pixels corresponding to an abnormal nozzle. Also, increasing the density of only pixels in the very vicinity of the unfilled pixels may increase the granularity.
Meanwhile, there are inkjet recording apparatuses employing a multi-dot technology that enables varying the droplet size. In such inkjet recording apparatuses, it may happen that ink droplets of a certain size cannot be jetted but ink droplets of other sizes can be normally jetted. The method disclosed in patent documents 3 through 7 is based on the binary representation of pixels and is therefore not applicable to the multi-dot technology where the droplet size is variable. It is of course possible to treat nozzles incapable of jetting ink droplets of one or more sizes as abnormal nozzles. However, in this case, correction is applied even to pixels to be formed with ink droplets of sizes that the abnormal nozzles can jet correctly. This in turn may reduce the quality of an image.
Patent documents 8 and 9 propose still another correction method that employs a multilevel error diffusion process supporting the multi-dot technology. In the proposed method, droplet sizes of pixels around pixels corresponding to an abnormal nozzle are changed to cover a defect caused by the abnormal nozzle or to compensate for droplets with incorrect sizes jetted by the abnormal nozzle.    [Patent document 8] Japanese Patent Application Publication No. 2006-115431    [Patent document 9] Japanese Patent Application Publication No. 2006-173929
The disclosed method using the multilevel error diffusion process makes it possible to form dots to cover a white stripe caused by an abnormal nozzle while achieving a natural balance with surrounding normal pixels by appropriately controlling sizes and arrangements of droplets.
Meanwhile, in inkjet recording, the quality of a recorded matter varies greatly depending on physical properties of ink and a recording medium used. Although the above method makes it possible to achieve a natural arrangement of ink droplets on a recording medium using the multilevel error diffusion process, i.e., to optimize the two-dimensional arrangement of ink droplets, the influence of increasing the droplet sizes such as the influence on the penetration of ink into the recording medium is not considered in the method.
Normally, the maximum droplet size is determined such that a surface area of a recording medium can be completely covered using droplets with the maximum droplet size (i.e., a droplet size that can fill a pixel (the area assigned to each pixel) or form a “fill pattern” as shown in FIG. 25(a)). The maximum droplet size may be set at a value greater than a droplet size necessary to fill a pixel. However, since the degree of penetration of an ink into a recording medium varies depending on the absorption characteristics of the ink and the recording medium, using droplets with such a large droplet size (or a large amount of ink) may in some cases cause bleeding and reduce the quality of an edge portion (or at a color boundary) of an image and may also affect color development.
Such problems caused by excessive amount of ink are side effects (secondary problems) caused by a correction process performed to cover a white stripe. Thus, the above related-art method employing the multilevel error diffusion process may cause such side effects.